Punchingis another function of cutting dies. Punching is the cutting of a slug from the sheet metal stock to produce a hole or slot. Cutting dies are also used to trim excess metal from around a formed part.
Hole punching and other cutting operations require specific and carefully aintained clearances between the punch (male component) and the die (female component). The setting of the required clearances is determined by both the stock thickness and temper. In general, die clearances increase as the stock thickness increases. The depth of punch penetration into the sheet metal stock will also increase as softer stock is used.
Forming is a general term used to describe a stamped part whose shape and
contour is reproduced directly from the shape and contour of a die set. The main
forming operations accomplished with press mounted dies are:
• drawing
• bending
• flanging
• hemming
Drawing, or draw forming, involves forcing a blank deeply into a die cavity and shaping it into the shape and contour of the punch face and sides. Without sufficient formability qualities, drawn blanks are subject to wrinkling, thinning, and fracturing. Draw forming requires an addition to the die set called a blankholder. The function of the blankholder, usually a ring through which the
punch and ram pass, is to control the metal flow as it is forced into the die cavity. In practice,the blankholder must exert less pressure against the blank than the punch, so metal can flow into the die; yet it must exert enough pressure to prevent the material from wrinkling.
Bending is a relatively simple forming operation which provides rigidity and shape to sheet metal parts. Similar to bending is flanging. However a flange is significantly smaller in dimension than the rest of the part. The functions of a flange include:
• giving a more finished appearance
• rigidity
• edge strengthening
• providing a fastening or attachment surface
Hemming is the folding over of a short flange upon itself to form a smooth, rounded edge and to facilitate the attachment of mating parts.
Multiple stamping operations may be performed within a single die, or at a number of die stations within a die set and with a single stroke of the press.
Single station dies can be either compound dies or combinations dies. A compound die performs basic cutting operations such as blanking and hole punching to produce parts. Combination dies combine shaping and forming functions with cutting operation to manufacture parts.
Multiple station dies are arranged so that a series of sequential operations are accomplished with each press stroke. Two die types are used; progressive and transfer. With progressive dies, coil stock is fed into the press. Individual stampings are connected with a carrier strip as they progress through the
various die operations and are ultimately separated and then discharged from the
press. In transfer die operations individual stock blanks are mechanically moved from die station to die station within a single die set. Large stampings are
done with tandem press lines where the stock is moved from press to press in which specific operations, such as drawing or trimming, are performed.
The resistance of the sheet metal stock to the forces exerted by the moving dies creates friction. For this reason, lubrication is vital for successful sheet metal forming. A lubrication's function is to minimize contact between the tooling and the work piece. This results in reduced tonnage requirements, longer tooling life, and improved product quality.
Lubricants range from light mineral oils to high viscosity drawing compounds. They may be oil base, water soluble, or synthetic materials. These lubricants may be applied in a variety of ways, including:
• manually by roller or brush
• drip
• machine roller
• spraying
• flooding
Die making is as much of an art as a science. When all the dynamics of stamping are taken into account, the resulting part may not meet all expectations. To help fine tune the stamping process and finalize die design, die makers use an analytical tool called Circle Grid Analysis, or CGA. The application of CGA involves the etching of a pattern of small circles on the surface of the blank. This pattern deforms along with the blank as it is formed, providing point-to-point calculations of the deformation that occurred. Analyzing this stamped grid pattern suggests
the location and type of rework that must be performed on the dies to produce easily manufactured parts. The CGA process is repeated on the die until an acceptable part is produced.
